Traditional models of learning have either been linear, viewing learning as a steady rate of growth over time, or staged, which
views learning as a stepped process characterized by sudden leaps to new plateaus in understanding. The Threshold Concept, however, offers a new
model for understanding the process of learning. As Atherton says, "It would be more realistic to depict [learning] as like a fell-walk. It has
elements of slope and step, but also it has a series of summits; one of the most frustrating but also rewarding aspects of a walk is to breast a summit
which you thought was the top of a hill, but which really just concealed another behind it. But it was nonetheless an intermediate peak, and from it you
can see both ahead and behind in a new way...In learning terms, that summit you have just breasted is a 'threshold concept.'"2

Identifying threshold concepts can help guide the course design process by identifying those learning objectives which are most essential to avoid "overstuffing"
the curriculum2. Threshold Concepts often share several specific features, including3:

Transformative: Once understood, a threshold concept changes the way in which the student views the discipline.

(often) Troublesome: Threshold concepts are likely to seem incoherent, counterintuitive, or foreign to students at first and may present difficulty in mastering the idea.

(often) Irreversible: Given their transformative potential, threshold concepts are likely to be irreversible or difficult to unlearn.

Integrative: They are likely to serve as connection points, bringing together various aspects of the topic that seemed disparate or unrelated to students.

Discursive: Crossing a threshold is likely to enable a student to incorporate an enhanced and extended use of language to express, reflect upon, and communicate learned ideas.

Once identified, threshold concepts serve as "jewels in the curriculum," those pivotal focus points for students.1
They can also remind instructors to reflect on their own learning process and develop empathy for the emotional challenge that struggling to master difficult ideas can
often present. Faculty will likely be encouraged to make space for tolerating confusion in their students and build in more recursiveness in their curriculum. "In
short, there is no simple passage in learning from "easy" to "difficult"; mastery of a threshold concept often involves messy journeys back, forth
and across conceptual terrain."1

References

2. Threshold Concepts, Flanagan, M. (2016). Threshold Concepts: Undergraduate Teaching, Postgraduate Training, Professional Development and School Education (A Short Introduction and a Bibliography). University College London.